Slurry recovery process

ABSTRACT

Frac sand from a well completion operation is transported to a recycling facility by truck. The frac sand is passed into a washing facility where is it separated from frac sand carrier residue, clays, drilling mud and metal particles. The wash water used in the washing facility is settled and re-used. Mud from the settled wash water is removed and re-used in drilling operations. The frac sand is removed from the washing facility by an inclined dehydration screw, and discharged into a counterflow hot air drying facility. Dust from the drying facility is collected and removed. Hot frac sand from the drying facility is passed through a chiller, and collected. The collected material is sized, and stored for re-use.

FIELD OF THE INVENTION

[0001] This invention relates generally to a process for the recoveryand recycling of slurry materials produced in well drilling operations,such as frac sand or aqueous liquid for transporting frac sand, or both.

BACKGROUND OF THE INVENTION

[0002] A number of processes employ sand having specific qualities. Insome cases the particular sand is only available from a small number ofsources, and may have to be transported over relatively long distances,at significant expense, to the site where it is to be employed. The sandmay have specific qualities with respect to size, hardness or strength,resistance to chemical attack, roundness, or other properties that arerequired for the particular process in question.

[0003] One such process involves the introduction of frac sand intowells under pressure. In the operation of a well, such as an oil or gaswell, it may generally be desirable for the stratum, or strata, of theproducing zone from which the oil or gas is to be extracted to have anumber of cracks or fissures running generally away from the well bore.The general premise is that the larger the cracks, and the farther theyrun away from the well bore, the easier it is for the gas or oil in thestratum to migrate toward the well bore, whence it can flow, or bedrawn, to the well-head at the surface.

[0004] One way to promote, or enhance, this process is to encourage thecreation of a large number of cracks, and to try to force cracks in thestratum to open up, to lengthen, and to stay open so that the oil or gascan flow out. One method of doing this is to raise the pressure in thewell, so that the adjacent cracks may tend to open up, and to force a“proppant” into the cracks the have been opened, so that when the raisedpressure is removed, the cracks may tend to stay propped open. It isimportant that the propant be strong enough to keep the crack in thestratum open, and yet not fill the crack so tightly that the path formigration of oil or gas be thereby blocked. That is, the propantmaterial must also let the oil and gas flow out.

[0005] One type of proppant material widely used in the oil and gasindustry is a type of silica sand of a particular range of size androundness. The roundness of the sand is advantageous since the grains,when stacked together, are interstitially porous. That is, the pathsbetween the stacked, rounded grains, are sufficiently porous to permitthe oil and gas to migrate through the spaces between the grains towardthe well bore. It is not advantageous to use sharp particles that stacktightly together. The desirable, rounded sand of appropriate size isavailable from only a relatively small number of suppliers, and tends tobe relatively expensive. When the propant insertion process iscompleted, the residue in the well bore is produced and, heretofore,discarded. Inasmuch as this residue contains a significant amount offrac sand propant, and inasmuch as frac sand is relatively expensive tobuy and transport, it would be advantageous to recover frac sand fromthe residue produced from the well.

[0006] However, frac sand has not been recovered in the past for anumber of reasons. First, the residue produced from the well includesnot only frac sand, but may also include undesirable materials such asmetal particles, drilling mud, and liquid frac sand carrier. The metalparticles may tend to be steel filings worn from the drill string by theabrasive action of the frac sand, or other abrasive material, within thedrill string itself. Alternatively the iron particles may be abradedfrom the casing of the bore, or from particles separated from the casingby a well perforating gun. The drilling mud tends to be a residue leftover from the drilling of the well in the first place. The liquid fracsand carrier arises from the frac sand process itself.

[0007] The frac sand process involves mixing dry, clean frac sand with acarrier. It is important that the frac sand be distributed reasonablyevenly in suspension in the carrier, as opposed to settling out, so thatit will be relatively well dispersed for entering a multitude of cracksalong the producing zone of the well bore. Therefore, at the time ofmixing at the well-head, the carrier has the form of gel, or gel likesubstance in which the frac sand is suspended. The mixture is thenforced down the well bore under high pressure. The pressure may tend toforce the cracks in the producing stratum to open, and in opening toadmit both the gel and the frac sand suspended in the gel. When thepressure is reduced to the pre-existing level, the cracks in theproducing stratum attempt to close, but are held (that is, propped) openby the frac sand. Once the frac sand is in place, it is desirable thatthe gel carrier be removed. Therefore the gel has a limited duration,and after a time it returns to a thin, predominantly watery liquid form.As such it drains back into the well bore, and leaves most of theproppant sand behind. The excess proppant also remains in the well bore.

[0008] In due course, the material that drains into the well bore ispumped out, typically as a slurry of several components, namely: theliquid carrier; the excess frac sand; left-over drilling mud; andwhatever else may have collected in the bore. In former times, theslurry produced from a new well at completion was extracted from thewell and sprayed over terrain adjacent to the well head. This is notnecessarily a desirable method of disposal, and has fallen out of favourin more recent times. The liquid carrier of the slurry from thecompleted well may well be a brine, and it is not desirable to sprayvarious chlorides over adjacent agricultural lands. It would bepreferable to recover the frac sand and to re-inject the iron ladenaqueous solution back into the ground at an injection well.

[0009] While it is desirable to re-use the frac sand, it is undesirableto reintroduce into the well proppant sand that is contaminated withmetal particles, since the metal particles may tend to then be forcedinto, and to block, the interstitial spaces in-between the sand granulesthat are needed to permit oil or gas to drain out. Similarly, it isundesirable to reintroduce proppant that is contaminated with drillingmud, since the drilling mud (and any undersized, cracked particles ofproppant) will tend to block, i.e., stopper up, the cracks as well. Itis thought that it may also be undesirable to re-introduce proppant thatis contaminated with used carrier fluid. It would be desirable, then, toextract the frac sand residue, remove contaminants such as metalparticles, drilling mud, broken or undersized particles of frac sand, orused carrier fluid, and dry the recovered frac sand so that it is readyfor re-use. It would also be desirable to re-condition the aqueoussolution that carries the frac sand to the surface to permit it to bedischarged benignly as approved, or used in other operations, such asinjection wells.

[0010] Injection wells have been used to extend production from existingwells. It may be that a producing well produces a mixture of water andoil. At the surface the water is separated from the oil. The oil istransported to a refinery, and the water remainder is then disposed. Oneform of disposal for the water is to re-inject it under pressure backinto the oil or gas bearing stratum whence it came. The reinjected wateris chemically treated before reinjection, the chemical treatment processtypically including a biocide. Over a period of years, the repeatedaction of treating and re-injecting water may tend to result in a buildup of sulphuric acidity in the water pumped from the producing well.

[0011] At the same time, the waste water from a new well frac sandcompletion process may tend to transport abraded iron particles that maytend to be small and to have a relatively high ratio of surface area tomass. Further, the solubility of iron in a salt solution may tend to behigher than in fresh water. If the aqueous solution of chlorides andiron from the new well, and low concentration sulphuric acid solutionfrom the producing well are mixed, a black iron sulphate precipitate maytend to form. It is undesirable to direct this precipitate back into theinjection well. First, the pumps of the injection well may be damaged bythe precipitate, and second, over a period of time it is not desirableto clog the pores of the well with solid precipitate. In that light, itis desirable to treat the waste water by-product solution of the fracsand cleaning process to discourage the black precipitate from forming,and to mechanically filter the waste water output by passing it througha relatively fine filter. In particular, it may be desirable tointroduce a modest concentration of citric acid, such as in a solubleanhydrous powder form, into the frac sand waste water output to form asoluble ferric citrate from which black ferric sulphate may have lesstendency to form when the solutions are mixed.

[0012] Washing and drying systems for sand may tend to use washwater,and produce a quantity of dust. It would be advantageous to recycle atleast part of the wash water, and thereby to reduce overall waterconsumption as compared to using only fresh water. It would also beadvantageous to employ a dust collector, or collectors, to reduce theamount of dust given off in the process.

SUMMARY OF THE INVENTION

[0013] In an aspect of the invention there is a process comprising thesteps of collecting frac sand after it has been used in a well bore,washing the frac sand, drying the frac sand and accumulating the washedand dried frac sand for re-use.

[0014] In an additional feature of that aspect of the invention, theprocess includes the step of segregating the frac sand by size beforeaccumulating the washed and dried frac sand for re-use. In anotheradditional feature, the process includes the step of mixing the washedand dried frac sand with a suspension agent and injecting the suspendedfrac sand into a well. In still another additional feature, the processfurther includes the step of collecting includes the step of collectinga slurry, a portion of the slurry being frac sand, settling the slurry,and decanting free liquid from the settled slurry.

[0015] In another aspect of the invention, there is a process forrecovering used frac sand from a well, comprising the steps ofextracting a slurry from the well, the slurry including at least fracsand and a carrier liquid, separating free liquid from the slurry toleave a first remainder, washing the first remainder in a washingfacility, extracting a second remainder of wet solids from the washingfacility, drying the wet solids, separating re-usable, dried frac sandfrom other solids and accumulating the re-useable frac sand.

[0016] In an additional feature of that aspect of the invention, thestep of washing includes introducing the first remainder into a tub. Inanother additional feature, the step of washing includes directing washwater at the first remainder. In still another additional feature, theprocess further includes the step of re-cycling at least a portion ofthe wash water. In yet another additional feature, the step of recyclingincludes the step of settling the used wash water. In still yet anotheradditional feature, the step of settling the used wash water is followedby the step of decanting clear wash water for re-use.

[0017] In another additional feature, the step of setting includesintroducing used wash water into a settling tank, allowing solids tosettle out, and removing settled solids from the tank. In still anotheradditional feature, the process further includes the step of chemicallytreating the wash water before directing it to the washing facilityagain. In a further additional feature, the process includes discardinga portion of the used wash water. In still yet a further additionalfeature, the process includes the step of directing an outflow of washwater from the washing facility along a path having at least one weir,and passing the flow over at least one weir.

[0018] In yet another additional feature, there is a settling zoneupstream of the weir, and the process includes the step of passing theoutflow through the settling zone on the way to the weir. In stillanother additional feature, there is a cascade of a plurality of weirsalong the path, and the process includes removing precipitated materialfrom upstream of the weirs. In yet another additional feature, theprocess includes the step of directing the outflow to a settling tankdownstream of the weirs. In still another additional feature, theprocess includes decanting wash water for re-use from the settling tank.In another additional feature, the step of extracting includes raisingthe frac sand with an inclined screw.

[0019] In yet another additional feature, the step of drying includesthe step of introducing at least a portion of the second remainderextracted from the washing facility into a rotating drum and introducinghot air into the drum to dry the second remainder. In a furtheradditional feature, the step of drying includes passing exhaust air fromthe drum through a dust collector. In yet a further additional feature,the step of washing includes magnetic separation of metal particles fromthe slurry.

[0020] In yet another additional feature, the process includes the stepof screening the solids to remove oversize particles. In still anotheradditional feature, the process includes the step of screening thesolids to exclude undersized particles. In a further additional feature,the process includes the step of screening the solids to exclude bothoversize and undersize particles. In still yet a further additionalfeature, the process includes the step of screening the solids by size,and the step of screening includes the step of collecting dust arisingfrom the step of screening.

[0021] In a further aspect of the invention, there is a process forrecovering used frac sand from a well completion operation. The processincludes the steps of extracting a slurry from the well, the slurryincluding at least frac sand, drilling mud, and a liquid carrier;passing at least a portion of the slurry through a washing facility toseparate the drilling mud and the liquid carrier from at least a portionof the frac sand; removing at least the portion of the frac sand fromthe washing facility, drying at least the portion of the frac sand; andpassing at least the portion of the frac sand through a sizing apparatusto yield a re-usable remainder.

[0022] In a still further aspect of the invention, there is a processfor recovering re-usable frac sand from a hydrocarbon well completionoperation. The process includes the steps of extracting a slurry from awell bore of a well, the slurry including a carrier liquid, frac sand,and drilling mud; passing at least a portion of the slurry into awashing facility; washing the drilling mud off the frac sand to yield afrac sand rich remainder; extracting the frac sand rich remainder fromthe washing facility; passing the frac sand rich remainder through adryer; and passing the frac sand rich remainder through a sizingapparatus to yield re-usable washed, dried and sized frac sand.

[0023] In an additional feature of that aspect of the invention, thestep of passing at least a portion of the slurry into the washingfacility includes a step of passing the portion of the slurry through aninput sizing apparatus to reject oversize solids, and then washing theportion of the slurry, less the oversize solids. In another additionalfeature, at least some of the slurry is at least partially settled toyield a free liquid portion and a solid rich portion, and the processincludes the step of decanting the free liquid portion beforeintroducing the solid rich portion of the slurry into the washingfacility.

[0024] In a further additional feature, an input sizing apparatus ismounted to receive the portion of the slurry, the input sizing apparatusincluding a reciprocating screen. The washing facility includes a waterdelivery apparatus mounted to spray water over the portion of the slurryand the reciprocating screen. The process includes the steps of passingat least the portion of the slurry through the input sizing apparatus toreject off-spec solid material, spraying water on the solid rich portionas the solid rich portion is passed through the reciprocating screen.

[0025] In a yet further additional feature, the washing facilityincludes a magnetic element and the process includes the step ofoperating the magnetic element to extract ferro-magnetic particles.

[0026] In another feature, the washing facility includes a washwatersupply, an outflow, and a settling system, the outflow being located todischarge into the settling system. The process includes the steps ofproviding washwater from the washwater supply to wash the drilling mudfrom the frac sand; and transporting at least a portion of the drillingmud in suspension in sand washwater through the outflow into thesettling system. In still another feature, the process includes thesteps of accumulating a sludge of drilling mud in the settling systemand removing the accumulated sludge of drilling mud. In a furtherfeature, the process may include re-using the drilling mud sludge in adown-hole drilling operation. In a still further feature, the outflowsystem includes a plurality of settling tanks arranged in series. Afirst of the settling tanks is located to receive the discharge from theoutflow of the washing facility. A second of the settling tanks islocated to receive a discharge from the first settling tank. A weir islocated between the first and second settling tanks. The discharge fromthe first settling tank flows across the weir, and the process includesthe step of skimming water from the settling system and re-using atleast a portion of the water skimmed from the settling system.

[0027] In an additional feature, the process further includes the stepof re-directing at least a portion of the water skimmed from thesettling system to the washing facility. In another additional feature,the process further includes the step of directing at least a portion ofthe water skimmed from the settling system to an injection well.

[0028] In yet another feature, the washing facility includes a bath anda screw mounted at least partially within the bath, the screw beingoperable to urge the frac sand rich remainder from the bath toward thedryer. In a further additional feature, a chute is mounted to direct thefrac sand rich remainder discharged from the screw to the dryer, and theprocess includes the stop of directing air from the dryer into thechute.

[0029] In still another feature, a dust extraction apparatus isconnected to the dryer and the process includes the step of directinghot air exhaust from the dryer to through the dust extraction apparatus.In a further feature, a cooling apparatus is connected to receive atleast a portion of the hot, dried frac sand rich remainder from thedryer and the process includes the step of passing the portion of thehot, dried frac sand rich remainder through the cooling apparatus. Instill yet another feature, the second, or output, sizing apparatusincludes a means for rejecting oversized particles, a means forrejecting undersized particles, and a discharge for “on-spec” particlesand the process includes the step of accumulating and storing the“on-spec” particles for re-use.

[0030] In a still further aspect of the invention, there is a processfor recovering re-usable frac sand from a hydrocarbon well completionoperation. The process includes the steps of extracting a slurry from awell bore of a well, the slurry including a carrier liquid, frac sand,and drilling mud; settling the slurry to produce a free liquid portionand a solid rich portion; removing the free liquid portion; passing thesolid rich portion through a first sizing apparatus to reject oversizesolids; passing the solid rich portion, less the rejected oversizesolids, into a washing facility; washing the drilling mud off the fracsand of the solid rich portion to yield a frac sand rich remainder;extracting the frac sand rich remainder from the washing facility;passing the frac sand rich remainder through a dryer; and passing thefrac sand rich remainder through a second sizing apparatus to yieldre-usable washed, dried and sized frac sand.

[0031] In still another aspect of the invention, there is an apparatusfor re-cycling used frac sand. The apparatus has machinery operable tocollect used frac sand, and to transport the used frac sand; a washingfacility for washing the frac sand, located to receive transported fracsand; a drying facility for drying the frac sand mounted to receivewashed frac sand from the washing facility; a sizing machine forsegregating the frac sand from off-spec material, the sizing machinebeing located to receive the frac sand from the drying facility; and astorage container for holding frac sand, the storage container beinglocated to receive on-spec frac sand from the sizing machine.

[0032] In a still further aspect of the invention, there is the use forre-cycling of frac sand of an apparatus having machinery operable tocollect used frac sand, and to transport the used frac sand; a washingfacility for washing the frac sand, located to receive transported fracsand; a drying facility for drying the frac sand mounted to receivewashed frac sand from the washing facility; a sizing machine forsegregating the frac sand from off-spec material, the sizing machinebeing located to receive the frac sand from the drying facility; and astorage container for holding frac sand, the storage container beinglocated to receive on-spec frac sand from the sizing machine.

[0033] In still another aspect of the invention there is a process forthe treatment of slurry waste water from a well completion operation.The process includes the step of extracting a slurry from a well bore.The slurry includes at least an aqueous liquid, and solids suspended inthe liquid; the solids including sand; iron in at least one form chosenfrom the set of: (a) iron in solution in the aqueous liquid, and (b)iron particles suspended in the aqueous liquid amongst other solids. Theprocess further includes the steps of separating the aqueous liquid fromat least the majority of the solids by using a mechanical separationapparatus to produce a remainder including at least some of the aqueoussolution and some of the iron; converting at least some of the iron inthe remainder to a compound form; mechanically filtering the remainderto remove iron particles from suspension to produce a treated outputsolution; mixing the treated output solution with a solution ofre-cycled hydrocarbon field production water; and re-injecting themixture of the treated output solution and the re-cycled hydrocarbonfield production water into a hydrocarbon producing stratum.

[0034] In an additional feature of that aspect of the invention, thestep of converting includes adding citric material to the remainder.

[0035] These and other aspects and features of the invention may beunderstood with the assistance of the FIGS and description as providedhereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 shows a plan view of a sand recovery installation accordingto the present invention;

[0037]FIG. 2A is a first portion of a pictorial schematic diagram of thesand recover installation of FIG. 1;

[0038]FIG. 2B is a second portion of a pictorial schematic diagram ofthe sand recover installation of FIG. 1;

[0039]FIG. 2C is a third portion of a pictorial schematic diagram of thesand recover installation of FIG. 1;

[0040]FIG. 2D is a fourth portion of a pictorial schematic diagram ofthe sand recover installation of FIG. 1; and

[0041]FIG. 2E is a fifth portion of a pictorial schematic diagram of thesand recover installation of FIG. 1; and

[0042]FIG. 3 shows a block diagram representation of a process employingthe installation of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The description which follows, and the embodiments describedtherein, are provided by way of illustration of an example, or examplesof particular embodiments of the principles of the present invention.These examples are provided for the purposes of explanation, and not oflimitation, of those principles and of the invention. In the descriptionwhich follows, like parts are marked throughout the specification andthe drawings with the same respective reference numerals. The drawingsare not necessarily to scale and in some instances proportions may havebeen exaggerated in order more clearly to depict certain features of theinvention.

[0044]FIG. 1 shows a plan view, or map, of a frac sand recoveryinstallation, indicated generally as 20. It is located some distancefrom a well field 22 from which frac sand is to be recovered. A batteryof wastewater injection wells 24 is also located some distance away inan old, existing producing well field, 23 (FIG. 2B). Old, exitingproducing wells are indicated as 25. A road 26 links installation 20with well field 22 and injection wells 24. A pipe system 28 also linksinjection wells 24 and frac sand recovery installation 20.

[0045] Operation of the frac sand recovery installation 20 is controlledand directed from a control trailer, 21. Describing installation 20 inthe order in which material would be processed, input material arrivesat the site of sand recovery installation 20 by truck 30. The materialis generally obtained in one of two ways.

[0046] First, well 32 in producing field 22 has been subject to a wellcompletion treatment to introduce frac sand. That is, well 32, typicallybeing a new well, has been pressurised to open cracks in the adjacentmineral bearing stratum. Frac sand has been mixed with a gel carrier,and has been introduced under pressure into well 32, and forced underpressure into the cracks and fissures of the adjacent stratum. Then thepressure in the well has been reduced to its previous level. The carrierhas reverted to a liquid form, and has drained back into the well,leaving the majority of the frac sand in place as a proppant to keep thecracks open. The remainder of the frac sand, the liquid carrier, andother matter, such as drilling mud with any metal shards or particlesabraded from the drill string, collect in the well bore sump. Thematerial from the well bore has been collected by producing it from thewell using collection apparatus in the nature of pumps and collectiontanks as may be suitable, and transferred to the frac sand recovery siteusing transport machinery such as truck 30. Truck 30 is most typically avacuum truck for transporting the collected material in the form of aslurry.

[0047] Second, the collected material may have been recovered from anold pit or dump 34. That is, formerly, the slurry produced from wellsafter introduction of proppant has usually been produced from the well,and then discarded, typically being collected at the well head and thendischarged into a pit or dump 34 from which the liquid eventuallyevaporates or drains away. As the opportunity to re-cycle potentiallyvaluable frac sand is made available, the formerly discarded materialcan be mined from existing dump sites. Material collected, such as bymining from a dump site, can be transported in either vacuum truck 30 ifwet, or in a gravel truck 36 if drained, as may be appropriate. Whilepreferred, it is not necessary that the free water be drawn off.Decanting off the free water first may tend to reduce the quantity ofliquid poured into the bins noted below.

[0048] If the material to be treated arrives in a vacuum truck 30, as ismost typical, it will generally have settled during the trip, and vacuumtruck 30 may be equipped with a vibration unit to enhance this settling.This may result in a first portion of the water, or watery liquid, invacuum truck 30 forming a layer of “free water” from which the solidshave settled out. As such, as a preliminary optional step, the freewater is decanted off the slurry in vacuum truck 30 to a sloop tank 38.This leaves a first remainder in truck 30, the first remainder includingthe solids and the balance of the liquid remaining after the free waterhas been decanted off. The free water from well 32 may tend typically tobe a salty chloride brine with iron in solution, or possible an amountof very fine iron particles in suspension.

[0049] Sloop tank 38 is a semi-buried open top tank having a liner -that is, tank 38 is equipped with an external secondary containmentliner and has a monitoring well 40 placed at one end of the tank.Monitoring well 40 can monitor the interstitial space 42 between thetank and the liner. Water collected in sloop tank 38 can be removed fortreatment elsewhere, can be directed into the wash-water systemdescribed below, or can be used elsewhere for other purposes, as may bepermitted, or may be discarded in an approved manner.

[0050] Once the free water has been removed, the solids rich remainderof the material in the vacuum truck 30 (or the material in gravel truck36, as may be) is weighed at weigh scale 44, adjacent to scale controltrailer 45, and emptied at either of a pair of first and second offloadpits 46, 48. Offload pits 46, 48 are also semi-buried tanks that eachhave a secondary containment liner and a leak detection system in thenature of an interstitial monitoring well, also indicated as 40, asnoted above. Pits 46, 48 (i.e., the semi-buried tanks) are sloped toprevent any free fluids from running out. A monitoring well 40 is againpositioned at the low point to permit effective leak detection. Truck30, (or 36, as may be), now empty, is re-weighed, and the differencebetween the full and empty weights calculated, that difference being theweight of wet sand material deposited in the offload pit.

[0051] The material from offload pits 46, 48, is transported to an inlethopper 52 of the washing facility, indicated generally as 50. Transportmay be by a conveyor or, as in the preferred embodiment, by a front-endloader 54.

[0052] A conveying device in the nature of a rotating sand screw, orauger 56 feeds material from hopper 52 onto a reciprocating (that is,vibrating) wash deck 58 mounted above a tub 60 of washing facility 50(FIG. 2A). In the preferred embodiment hopper 52 is a 10′×10′ hopper,and auger 56 is a 24 inch sand screw. A wash water delivery system 62includes nozzles or water jets 64 located to direct an inflow of washwater against the solids rich material fed from auger 56 onto wash deck58. The wet material, and the wash water, fall or drain, through a ½inch grid screen 66 of wash deck 58, and into tub 60. Oversized materialrejected by screen 66 such as stones inadvertently included in materialrecovered from old storage pits is discarded through a discharge chuteto an oversize bin 68. Bin 68 is periodically emptied as required.

[0053] Tub 60 has two exits. The first exit is by an inclineddehydrating screw 70, described below. The second exit is by an outflowpassage 72 leading to a settling system in the nature of a threecompartment, high level overflow rig tank 74 (FIG. 2B). Tub 60 has ahigh level weir 71 leading to passage 72. Rig tank 74 has a series, orset, of weirs 76 and 78 of decreasing height. Although two such weirsare shown, a greater number could also be used. There are respectivesettling, or low velocity, zones 80, 82, 84, in each of the threecompartments, in which a sludge of heavier solid particles 67 silts outover time. The outflow from tub 60 may tend generally to transport awaythe drilling mud in suspension, and the resultant accretion of sludge 67collected upstream of weirs 76, 78 in settling zones 80, 82, 84 islargely, if not entirely, drilling mud. The collected mud is routinelyremoved from each of the three compartments.

[0054] The wash water is recycled through rig tank 74, on average, up toabout four times. That is, the ratio of the flow through re-circulationpump 75 and re-circulation line 77 from the most downstream settlingzone, namely zone 84, back to washing tub 60 is about four times asgreat (+/−) as the flow out through pump 85 and pipes 86. The wash wateris skimmed off the rig tank and moved, (that is, pumped through pipes86), into one of three alternately selectable waste water holding tanks,88, 90, 92. Fines are allowed further time to settle in holding tanks88, 90 and 92. While one tank is selected, waste water is being settledin one or both of the others. Tanks 88, 90 and 92 are periodicallycleaned or flushed to remove accumulated fines.

[0055] The settled wastewater is treated for downhole compatibility withadditives as shown schematically at 87, drawn from a settled tank,whichever it may be. The relatively clean, treated output wastewater isthen passed through a filter 89 before being sent either to the batteryof injection wells 24 for re-injection into the mineral bearing stratum,or for other use, or for discard in an approved manner. The chemicaladditives may include citric acid in a soluble anhydrous powder formsuch as may tend to combine with the iron in solution in the wastewater.The quantity of citric additive is also modest, in one embodiment beingof the order of a quart per 400 bbl settling tank. The quantity ofadditive required may tend to vary according to the condition anddilution of the waste water as received from truck 30. Filter 89 maytypically have filter elements for trapping solid particles of greaterthan 20 microns in size, the trapped material being removed at 91. Thevolume of water directed to the battery wells, or discarded, is measuredand monitored for solid precipitants.

[0056] Treated, filtered wastewater sent by pipe 24 to old producingfield 23 is mixed with treated, re-cycled water obtained when thematerial produced from old well 25 is separated into oil 95 and water97. The recycled water fiom well 25 has been treated with biocide, andmay tend to have a somewhat sour, sulphuric acidic nature, depending onthe condition of well 25. The mixture is then injected into well 24under pressure, as provided by pump 93.

[0057] Inasmuch as the metal particles tend to be predominantly, if notwholly, steel from the drill string, they also tend to be ferro-magneticin nature. Metal particle removal apparatus, in the nature of magneticheads 94 mounted at the outflow lip of high level weir 71 of tub 60 andat weirs 76 and 78, where the flow is shallow, to attract the metalparticles that may have been entrained in the slurry. Magnetic heads 94are periodically removed and cleaned of accumulated metallic material.

[0058] Returning to tub 60, the second exit is by a lifting device inthe nature of inclined dewatering screw 70 mounted within an inclinedchannel, or casing 73 that is manufactured as an inclined extension ofthe main body of tub 60. The sand and wash water that fall throughscreen 66 fall on dewatering screw 70 and into tub 60. Over time, theuse of the wash water and the agitation of the sand by dewatering screw70 tends to clean the frac sand, such that the sand eventually raised bydewatering screw 70 and thus extracted from tub 60 is relatively cleanfrac sand.

[0059] The wet sand discharged from the upper end of dewatering screw 70is directed through a chute 96 into a drying facility, namely rotatingdrum dryer 100 fed from chute 96 by auger 97(Fig. 2C). In the preferredembodiment dryer 100 is 5 ft in diameter and 40 ft long. Dryer 100 issupplied with heated air by a burner 102. The inside of dryer 100 hasbaffles 104 that lift and turn the sand and allow it to fall in theheated airflow as the dryer drum 106 turns. The incline of the dryer issuch as to urge the sand gently from the elevated inlet end at chute 96to the somewhat lower outlet end at burner 102 over a period of time,such that the sand may tend to be dry by the time it reaches the exit.

[0060] In the preferred embodiment dryer 100 operates at about 700 F.Dryer 100 is a counter-flow dryer operating on a negative air system.That is, the direction of the airflow, from burner 102, through rotatingdrum 106, is generally in the opposite, or counter-flow direction to theprogress of the sand that enters from chute 96. The sand proceeds alongrotating drum 106 to a sand discharge auger 108 located adjacent to theair inlet opening 110 into which burner 102 is directed. The negativeair system, rather than blowing air into dryer 100, draws it throughdryer 100 at a negative pressure relative to ambient, by use of an airmover, or fan, in the nature of a centrifugal blower 112.

[0061] Blower 112 (FIG. 2D) is mounted to draw the heated air throughdrum 106, from inlet 110 at burner 102, as noted above, and to extractthe hot air exhaust and entrained dust from the end of dryer 100adjacent to the chute 96. The air and dust extracted are drawn firstinto a large, dry cyclone 114. Settled off-spec undersized sandparticles and fines are discharged from dry cyclone 114 through adischarge chute 116 for collection in a receptacle, or containment binin the nature of an above ground shale sloop 118, and either sent to awaste management facility or reused as may be appropriate.

[0062] The air exhaust from dry cyclone 114 is drawn through ducting 113into a wet dust collection apparatus, or scrubber 122. In a firstportion 115 of wet scrubber 122, the exhaust air from dry cyclone 114 isdrawn downwardly through a large chamber 117 in which it has relativelylow downward velocity. Water is drawn from a settling tank 124 through apump 119 and piping 120 to be sprayed as a mist from an array of nozzles121 into the passing still relatively warm, dry air. The now moistenedair, and any unevaporated spray, then passes through a throat leading toa check valve 123. Check valve 123 permits the wet air to pass into thesecond portion of scrubber 122, namely a bath, 127 that is filled withwater. When the pressure in bath 127 at check valve 123 is lower than inportion 115, the air, and entrained dust particles, will be drawn intobath 127, the air bubbling upward to the surface of the water. The waterlevel of bath 127 is maintained by a spring loaded release valve 129that operates to release water from bath 127 when the internal levelexceeds a “full” height. The released water is returned to tank 124. Wetdischarge from scrubber 122 passes through a solid discharge chute 131to be collected in settling tank 124. The moisture laden air exhaustfrom wet scrubber 122 is extracted through ducting 126 by blower 112 andis discharged to the atmosphere through a stack 133.

[0063] A lifting device in the nature of a bucket elevator 128 (FIG. 2C)transports the hot, dry sand from the discharge of dryer 100 to the topof a cooling facility in the nature of a counter-flow cooling tower 130.Tower 130 has a sand inlet at the top, and a sand discharge at thebottom. An array of baffles 132 inside tower 130 causes the sand to fallthrough the airflow several times before reaching the bottom. Ambient,relatively cool air is introduced at the base of tower 130 and is drawnupward to encounter the cascading, downward falling sand. The air isforced out the top of tower 130 and then through a dust collectorcyclone 134 by a blower 136. The fines from cyclone 134 are collected inan open ended tank 138, and are disposed of in a suitable manner.

[0064] The sand leaving the bottom end of cooling tower 130 is at amoderate temperature, in the range of 150 F., with variation dependingon the ambient temperature. The sand leaving through the discharge chuteof cooling tower 130 falls into a receptacle in the nature of an openended tank 140, where it collects. The sand is then transported to asizing facility in the nature of a system of moving screens, mostpreferably a rotary screener 142. Rotary screener 142 rejects remainingsmall stones and over spec sand. This leaves a remainder of washed,dried, on-spec sand that is transported by an elevator 144 to one offour storage receptacles in the nature of bins 146, 148, 150 and 152.This sand is ready to be loaded by a transport means such as movableconveyor 154 onto trucks, such as gravel truck 158, weighed on scale 44(empty and full, as above, to permit the difference to be calculated)and sent back to well completion sites for use as frac sand. Theoversize, off-spec sand from screener 142 is accumulated in a bin 156and then transferred, typically by truck, for landfill or for use insand and gravel operations more generally. Both the volume of materialstored in bins 146, 148, 150 and 152 and the weight of material sent outfrom recovery facility 20 are monitored, as for example when departingtrucks 158 with clean frac sand are weighed empty and full. The fracsand can then be used, as described above, by mixing with the gelcarrier, introduction under pressure into a well, and so forth.

[0065] In this system, fresh water is provided from an external source,such as a lake or river, and is introduced by means of a pump 160 andpipe 162 at the wet scrubber setting tank 124, from which it is forcedby pump 61 and line 63 pumped to wash water delivery system jets 64, asnoted above. The decanted water from sloop tank 38 is pumped into thefirst settling region, 80, of rig tank 74 by means of a pump 164 andpipe 166.

[0066] Embodiments of the invention have now been described in detail.Since changes in and or additions to the above-described best mode maybe made without departing from the nature, spirit or scope of theinvention, the invention is not to be limited to those details, but onlyby the appended claims.

I claim:
 1. A process comprising the steps of: extracting frac sand froma well bore; collecting the frac sand extracted from the well bore;washing the frac sand; drying the frac sand; sizing the frac sand; andaccumulating the washed, dried and sized frac sand for re-use.
 2. Theprocess of claim 1 wherein the process includes the step of segregatingthe frac sand by size before accumulating the washed and dried frac sandfor re-use.
 3. The process of claim 1 further including the step ofmixing the washed and dried frac sand with a suspension agent andinjecting the suspended frac sand into a well.
 4. The process of claim 1wherein the step of collecting includes the step of collecting a slurry,a portion of the slurry being frac sand, settling the slurry to yield afree liquid portion, and decanting the free liquid portion from thesettled slurry.
 5. A process for recovering used frac sand from a wellcompletion operation, said process including the steps of: collecting adeposit that includes frac sand and other materials; introducing atleast the frac sand into a washing facility; washing said frac sand;discharging at least a portion of said frac sand from said washingfacility to a drying facility; drying at least said portion of said fracsand; passing at least said portion of said frac sand through a sizingapparatus to yield a remainder of washed, dried, and sized frac sand forre-use.
 6. The process of claim 5 wherein the step of washing includesintroducing the frac sand into a tub.
 7. The process of claim 6 whereinthe step of washing includes directing wash water at the frac sand as itis falling into said tub.
 8. The process of claim 7 further includingthe step of re-cycling at least a portion of the wash water.
 9. Theprocess of claim 8 wherein the step of recycling includes the step ofsettling used wash water.
 10. The process of claim 9 wherein the step ofsettling the used wash water is followed by the step of decanting clear,settled, wash water for re-use.
 11. The process of claim 9 wherein thestep of setting includes introducing used wash water into a settlingtank, allowing solids to settle out, and removing settled solids fromthe tank.
 12. The process of claim 8 further including the step ofchemically treating the wash water for compatibility with recycledproducing well water before directing it to an injection well.
 13. Theprocess of claim 8 including discarding a portion of the used washwater.
 14. The process of claim 8 including the step of directing anoutflow of wash water from the washing facility along a path having atleast one weir, and passing said flow over said at least one weir. 15.The process of claim 14 wherein there is a settling zone upstream ofsaid weir, and said process includes the step of passing the outflowthrough said settling zone on the way to said weir.
 16. The process ofclaim 14 wherein there is a cascade of a plurality of weirs along saidpath, and said process includes removing precipitated material fromupstream of said weirs.
 17. The process of claim 16 wherein said processincludes the step of directing the outflow to a settling tank downstreamof said weirs.
 18. The process of claim 16 wherein said process includesdecanting wash water for re-use from said settling tank.
 19. The processof claim 5 including the step of raising said frac sand from the tubwith an inclined screw.
 20. The process of claim 5 wherein the step ofdrying includes the step of introducing at least a portion of said fracsand discharged from said washing facility into a rotating drum andintroducing hot air into the drum to dry the frac sand.
 21. The processof claim 20 wherein the step of drying includes passing exhaust air fromthe drum through a dust collector.
 22. The process of claim 5 whereinsaid step of washing includes magnetic separation of metal particlesfrom said slurry.
 23. The process of claim 5 wherein the processincludes the step of screening the solids to remove oversize particles.24. The process of claim 5 wherein the process includes the step ofscreening the solids to exclude undersized particles.
 26. The process ofclaim 5 wherein the process includes the step of screening the solids toexclude both oversize and undersize particles.
 27. The process of claim5 wherein said process includes the step of screening the solids bysize, and the step of screening includes the step of collecting dustarising from the step of screening.
 28. A process for recovering usedfrac sand from a well completion operation, said process comprising thesteps of: extracting a slurry from the well, the slurry including atleast frac sand, drilling mud, and a liquid carrier; passing at least aportion of the slurry through a washing facility to separate saiddrilling mud and said liquid carrier from at least a portion of saidfrac sand; removing at least said portion of said frac sand from saidwashing facility, drying at least said portion of said frac sand;passing at least said portion of said frac sand through a sizingapparatus to yield a re-usable remainder.
 29. A process for recoveringre-usable frac sand from a hydrocarbon well completion operation, saidprocess comprising the steps of: extracting a slurry from a well bore ofa well, the slurry including a carrier liquid, frac sand, and drillingmud; passing at least a portion of the slurry into a washing facility;washing said drilling mud off said frac sand to yield a frac sand richremainder; extracting said frac sand rich remainder from said washingfacility; passing said frac sand rich remainder through a dryer; andpassing said frac sand rich remainder through a sizing apparatus toyield re-usable washed, dried and sized frac sand.
 30. The process ofclaim 29 wherein said step of passing at least a portion of said slurryinto said washing facility includes a step of passing said portion ofsaid slurry through an input sizing apparatus to reject oversize solids,and then washing said portion of said slurry, less the oversize solids.31. The process of claim 29 wherein at least some of said slurry is atleast partially settled to yield a free liquid portion and a solid richportion, and said process includes the step of decanting the free liquidportion before introducing the solid rich portion of the slurry into thewashing facility.
 32. The process of claim 29 wherein: an input sizingapparatus is mounted to receive said portion of said slurry, said inputsizing apparatus including a reciprocating screen; said washing facilityincludes a water delivery apparatus mounted to spray water over saidportion of said slurry and said reciprocating screen; and said processincludes the steps of passing at least said portion of said slurrythrough said input sizing apparatus to reject off-spec solid material,spraying water on said solid rich portion as said solid rich portion ispassed through said reciprocating screen.
 33. The process of claim 29wherein said washing facility includes a magnetic element and saidprocess includes the step of operating said magnetic element to extractferro-magnetic particles.
 34. The process of claim 29 wherein saidwashing facility includes a washwater supply, an outflow, and a settlingsystem, said outflow being located to discharge into said settlingsystem, and said process includes the steps of providing washwater fromsaid washwater supply to wash said drilling mud from said frac sand; andtransporting at least a portion of said drilling mud in suspension insand washwater through said outflow into said settling system.
 35. Theprocess of claim 34 further comprising the steps of accumulating asludge of drilling mud in said settling system and removing theaccumulated sludge of drilling mud.
 36. The process of claim 35 furthercomprising re-using the drilling mud sludge in a down-hole drillingoperation.
 37. The process of claim 34 wherein said outflow systemsincludes a plurality of settling tanks arranged in series, a first ofsaid settling tanks being located to receive the discharge from saidoutflow of said washing facility, a second of said settling tanks beinglocated to receive a discharge from said first settling tank, and a weirlocated between said first and second settling tanks, said dischargefrom said first settling tank flowing across said weir, and said processincluding the step of skimming water from said settling system andre-using at least a portion of the water skimmed from said settlingsystem.
 38. The process of claim 37 wherein said process furthercomprises the step of redirecting at least a portion of the waterskimmed from said settling system to said washing facility.
 39. Theprocess of claim 37 wherein said process further comprises the step ofdirecting at least a portion of the water skimmed from said settlingsystem to an injection well.
 40. The process of claim 29 wherein: thewashing facility includes a bath and a screw mounted at least partiallywithin said bath, said screw being operable to urge said frac sand richremainder from said bath toward said dryer.
 41. The process of claim 40wherein a chute is mounted to direct said frac sand rich remainderdischarged from said screw to said dryer, and said process includes thestop of directing air from said dryer into said chute.
 42. The processof claim 29 wherein a dust extraction apparatus is connected to saiddryer and said process includes the step of directing hot air exhaustfrom said dryer to through said dust extraction apparatus.
 43. Theprocess of claim 29 wherein a cooling apparatus is connected to receiveat least a portion of the hot, dried frac sand rich remainder from saiddryer and said process includes the step of passing said portion of thehot, dried frac sand rich remainder through said cooling apparatus. 44.The process of claim 29 wherein said sizing apparatus includes a meansfor rejecting oversized particles, a means for rejecting undersizedparticles, and a discharge for “on-spec” particles and said processincludes the step of accumulating and storing said “on-spec” particlesfor re-use.
 45. A process for recovering re-usable frac sand from ahydrocarbon well completion operation, said process comprising the stepsof: extracting a slurry from a well bore of a well, the slurry includinga carrier liquid, frac sand, and drilling mud; settling said slurry toproduce a free liquid portion and a solid rich portion; removing thefree liquid portion; passing said solid rich portion through a firstsizing apparatus to reject oversize solids; passing said solid richportion, less said rejected oversize solids, into a washing facility;washing said drilling mud off said frac sand of said solid rich portionto yield a frac sand rich remainder; extracting said frac sand richremainder from said washing facility; passing said frac sand richremainder through a dryer; and passing said frac sand rich remainderthrough a second sizing apparatus to yield re-usable washed, dried andsized frac sand.
 46. An apparatus for re-cycling used frac sand, theapparatus comprising: machinery operable to collect used frac sand, andto transport the used frac sand; a washing facility for washing the fracsand, located to receive transported frac sand; a drying facility fordrying the frac sand mounted to receive washed frac sand from thewashing facility; a sizing machine for segregating the frac sand fromoff-spec material, the sizing machine being located to receive the fracsand from the drying facility; and a storage container for holding fracsand, the storage container being located to receive on-spec frac sandfrom the sizing machine.
 47. The use for re-cycling of frac sand of anapparatus comprising: machinery operable to collect used frac sand, andto transport the used frac sand; a washing facility for washing the fracsand, located to receive transported frac sand; a drying facility fordrying the frac sand mounted to receive washed frac sand from thewashing facility; a sizing machine for segregating the frac sand fromoff-spec material, the sizing machine being located to receive the fracsand from the drying facility; and a storage container for holding fracsand, the storage container being located to receive on-spec frac sandfrom the sizing machine.
 48. A process for the treatment of slurry wastewater from a well completion operation, the process comprising the stepsof: extracting a slurry from a well bore, the slurry including at leastan aqueous liquid, and solids suspended in the liquid; the solidsincluding sand; the slurry including iron in at least one form chosenfrom the set of: iron in solution in the aqueous liquid; and ironparticles suspended in the aqueous liquid amongst other solids;separating the aqueous liquid from at least the majority of the solidsby using a mechanical separation apparatus to produce a remainderincluding at least some of the aqueous solution and some of the iron;converting at least some of the iron in the remainder to a compoundform; mechanically filtering the remainder to remove iron particles fromsuspension; to produce a treated output solution; mixing the treatedoutput solution with a solution of re-cycled hydrocarbon fieldproduction water; and re-injecting the mixture of the treated outputsolution and the re-cycled oil field production water into a hydrocarbonproducing stratum.
 49. The process of claim 48 wherein said step ofconverting includes adding citric material to the remainder.
 50. Theprocess of claim 48 wherein said process includes the step ofencouraging separation of iron particles from said aqueous liquidmagnetically.